Back contact diffusion barrier layers for group ibiiiavia photovoltaic cells

ABSTRACT

The present invention provides for new ohmic contact materials and diffusion barriers for Group IBIIIAVIA based solar cell structures, which eliminate two way diffusion while preserving the efficient ohmic contacts between the substrate and the absorber layers.

BACKGROUND

1. Field of the Inventions

The present inventions generally relate to apparatus and methods ofsolar cell design and fabrication and, more particularly, to formingcontact layers and diffusion barriers for such solar cells.

2. Description of the Related Art

Solar cells are photovoltaic devices that convert sunlight directly intoelectrical energy. Solar cells can be based on crystalline silicon orthin films of various semiconductor materials, usually deposited onlow-cost substrates, such as glass, plastic, or stainless steel.

Thin film based photovoltaic cells, such as amorphous silicon, cadmiumtelluride, copper indium diselenide or copper indium gallium diselenidebased solar cells, offer improved cost by employing depositiontechniques widely used in the thin film industry. Group IBIIIAVIAcompound photovoltaic cells including copper indium gallium diselenide(CIGS) based solar cells have demonstrated the greatest potential forhigh performance, high efficiency, and low cost thin film PV products.

A conventional Group IBIIIAVIA compound solar cell can be built on asubstrate that can be a sheet of glass, a sheet of metal, an insulatingfoil or web, or a conductive foil or web. A contact layer such as amolybdenum (Mo) film is deposited on the substrate as the back electrodeof the solar cell. An absorber thin film including a material in thefamily of Cu(In,Ga)(S,Se)₂, is formed on the conductive Mo film.Although there are other methods, Cu(In,Ga)(S,Se)₂ type compound thinfilms are typically formed by a two-stage process where the components(components being Cu, In, Ga, Se and S) of the Cu(In,Ga)(S,Se)₂ materialare first deposited onto the substrate or the contact layer formed onthe substrate as an absorber precursor, and then reacted with S and/orSe in a high temperature annealing process.

After the absorber film is formed, a transparent layer, for example, aCdS film, a ZnO film or a CdS/ZnO film-stack is formed on the absorberfilm. The preferred electrical type of the absorber film is p-type, andthe preferred electrical type of the transparent layer is n-type.However, an n-type absorber and a p-type window layer can also beformed. The above described conventional device structure is calledsubstrate-type structure. In the substrate structure light enters thedevice from the transparent layer side. A so called superstrate-typestructure can also be formed by depositing a transparent conductivelayer on a transparent superstrate such as glass or transparentpolymeric foil, and then depositing the Cu(In,Ga)(S,Se)₂ absorber film,and finally forming an ohmic contact to the device by a conductivelayer. In the superstrate structure light enters the device from thetransparent superstrate side.

In standard CIGS as well as Si and amorphous Si module technologies, thesolar cells can be manufactured on flexible conductive substrates suchas stainless steel foil substrates. Due to its flexibility, a stainlesssteel substrate allows low cost roll-to-roll solar cell manufacturingtechniques. In such solar cells built on conductive substrates, thetransparent layer and the conductive substrate form the opposite polesof the solar cells. Multiple solar cells can be electricallyinterconnected by stringing or shingling methods that establishelectrical connection between the opposite poles of the solar cells.Such interconnected solar cells are then packaged in protective packagesto form solar modules or panels. Many modules can also be combined toform large solar panels. The solar modules are constructed using variouspackaging materials to mechanically support and protect the solar cellsin them against mechanical damage. Each module typically includesmultiple solar cells which are electrically connected to one anotherusing above mentioned stringing or shingling interconnection methods.

The conversion efficiency of a thin film solar cell depends on manyfundamental factors, such as the bandgap value and electronic andoptical quality of the absorber layer, the quality of the window layer,the quality of the rectifying junction, and so on. Since the totalthickness of the electrically active layers of the CIGS thin film solarcells is in the range of 0.5-5 micrometers, these devices are highlysensitive to defects. Even the sub-micron size defects may influencetheir illuminated I-V characteristics. A prior art problem associatedwith manufacturing CIGS thin film devices on stainless steel substrates,however, is the inadvertent introduction of defects into the devicestructure during the reaction of the absorber precursor. During thisstep, the constituents of the stainless steel substrate diffuse towardsthe absorber layer, and Se from the absorber precursor diffuses towardsthe stainless steel substrate. Despite the advantages of flexiblestainless steel substrates in CIGS solar cell applications, during thehigh temperature anneal, Se from the absorber precursor can diffusethrough the Mo contact layer toward the substrate, corrode the stainlesssteel and form FeSe compounds on the substrate, which cause electricalshunting defects. Such shunting defects introduce a shunting path,between the substrate and the absorber or the transparent layer throughwhich the electrical current of the device may leak. The shuntingdefects lower the fill factor, the voltage and the conversion efficiencyof the solar cells. Furthermore, the atoms of the stainless steel suchas iron (Fe) atoms can diffuse from the substrate into the CIGS absorberby diffusing through the Mo contact layer during the aforementioned hightemperature anneal or reaction process that forms the absorber. Thisunwanted material diffusion into the absorber significantly degrades theperformance of the absorber and the resulting device. One prior artprocess uses a chromium (Cr) layer to prevent Fe diffusion into theabsorber; however, Cr forms CrSe with Se from the absorber precursorduring the reaction step, and just like FeSe, Cr causes shuntingdefects.

SUMMARY

The present inventions provide for new ohmic contact materials anddiffusion barriers for Group IBIIIAVIA based solar cell structures,which eliminate two way diffusion while preserving the efficient ohmiccontacts between the substrate and the absorber layers.

In one aspect is described a method of manufacturing a Cu(In,Ga) Se₂thin-film solar cell, comprising: forming an intermediate layer on asurface of a flexible conductive substrate, the flexible conductivesubstrate including iron (Fe) species, the step of forming theintermediate layer comprising: forming a conductive barrier stack on thesurface of the flexible conductive substrate, the conductive barrierstack including a chromium (Cr) film applied to the surface of theflexible conductive substrate and a titanium nitride (TiN) filmdeposited on the Cr film; depositing a contact layer on the conductivebarrier stack; depositing a nucleation layer on the contact layer;depositing an absorber precursor including Cu species, In species, Gaspecies and Se species over the nucleation layer; applying heat to theabsorber precursor to form therefrom an absorber layer including aCu(In,Ga) Se₂ thin film compound, wherein the step of forming theabsorber layer causes some of the Fe species in the flexible substrateto diffuse towards the absorber layer and some of the Se species in theabsorber layer to diffuse towards the flexible conductive substrate, andwherein the conductive barrier stack inhibits diffusion of both the iron(Fe) species and the Se species across the conductive barrier stackduring the step of applying heat; disposing a transparent conductivelayer on the thin film absorber layer, the transparent layer including abuffer layer deposited on the thin film absorber and a transparentconductive oxide layer formed on the buffer layer; forming a topterminal on the transparent conductive layer, thereby resulting in theCu(In,Ga) Se₂ thin-film solar cell; and wherein after the step offorming, the conductive barrier stack continues to inhibit diffusion ofboth the iron (Fe) species and the Se species across the conductivebarrier stack.

In another aspect is described a solar cell, comprising: a flexibleconductive substrate including Fe species; a thin film absorber layerincluding Cu species, In species, Ga species and Se species formed overthe flexible conductive substrate layer; an intermediate layer disposedbetween the flexible conductive substrate and the thin film absorberlayer, the intermediate layer including: a conductive barrier stack onthe surface of the flexible conductive substrate, the conductive barrierstack including a chromium (Cr) film applied to the surface of theflexible conductive substrate and a titanium nitride (TiN) filmdeposited on the Cr film, wherein the conductive barrier stack inhibitsdiffusion of both the iron (Fe) species from the flexible conductivesubstrate across the conductive barrier stack and the selenium Sespecies from the thin film absorber layer across the conductive barrierstack; a contact layer deposited on the conductive barrier stack; anucleation layer deposited on the contact layer; a transparent layerdeposited on the thin film absorber layer, the transparent layerincluding a buffer layer deposited on the thin film absorber and atransparent conductive oxide layer formed on the buffer layer; and a topterminal formed on the transparent layer.

These and other aspects and advantages are described further herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic side view of an embodiment of a solar cell baseof the present invention;

FIG. 1B is a schematic side view of solar cell using the solar cell baseshown in FIG. 1A;

FIG. 1C is schematic perspective view of the solar cell shown in FIG.1B; and

FIG. 2 is a schematic side view of another embodiment of a solar cellbase of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments described herein provide solar cellmanufacturing methods and device structures to prevent unwanted materialdiffusion between a metallic substrate and a Group IBIIIAVIA thin filmabsorber of a solar cell during the manufacture of the solar cell. Inone embodiment, an intermediate layer, including multiple conductivematerial films, is disposed between a stainless steel substrate and anabsorber layer of a solar cell. The absorber layer may be aCu(In,Ga)(Se,S)₂ or CIGSS compound thin film which is formed byannealing (reacting) an absorber precursor including Cu, In, Ga and Se,and optionally S at temperature range of about 400-600° C. in a reactor.Accordingly, during the reaction, the intermediate layer of the presentinvention inhibits or minimizes both unwanted diffusion mechanisms,namely, the iron (Fe) diffusion from the stainless-steel substrate tothe absorber layer and the selenium (Se) diffusion from the absorberlayer to the stainless steel. In one embodiment, a first intermediatelayer includes: a diffusion barrier stack deposited on the stainlesssteel substrate; a contact layer, such as a Mo layer, deposited on thediffusion barrier stack; a nucleation layer, such as a Ru layer,deposited on the contact layer; and a seed layer, such as a Cu layer,deposited on the nucleation layer. The diffusion barrier stackpreferably includes a metal barrier layer, such as a Cr layer, depositedon the stainless steel substrate and a metal nitride barrier layer, suchas a TiN layer, deposited metal barrier layer. In another embodiment, asecond intermediate layer preferably includes: a diffusion barrier stackincluding a Cr layer deposited on the stainless steel substrate and afirst TiN layer deposited on the Cr layer; a first contact layer, suchas a first Mo layer, deposited on the diffusion barrier stack; a secondTiN layer deposited on the first contact layer; a second contact layer,such as a second Mo layer, deposited on the second TiN layer; anucleation layer, such as a Ru layer, deposited on the second contactlayer; and a seed layer, such as a Cu layer, deposited on the nucleationlayer.

FIG. 1A shows a base 100 including a flexible substrate 102 having afront surface 104A and a back surface 104B, and an intermediate layer105 disposed on the front surface of the flexible substrate 102. Theintermediate layer 105 preferably includes multiple conductive films. Inone embodiment, the intermediate layer 105 includes: a diffusion barrierstack 103, a contact layer 110, a nucleation layer 112, and a seed layer114. In one example, the diffusion barrier stack 103 preferablyincludes, but is not limited to, a metal barrier layer 106 deposited onthe stainless steel substrate and a metal-nitride barrier layer 108deposited on the metal barrier layer 106. The metal barrier layer 106may be a Cr layer or may be Cr alloy such as CrMo, or multiple layers ofCr and Cr alloys. The thickness of the metal barrier layer 106 may be inthe range of 50-100 nm, preferably 80-100 nm, and more preferably 95-100nm. The metal nitride barrier layer 108 may include titanium-nitride(TiN), tantalum nitride (TaN), or tungsten nitride (WN), and may befully or partially made of TiN or a combination of the above nitrides.The thickness of the metal-nitride barrier layer 108 may be in the rangeof 200-400 nm, preferably 250-350 nm, and more preferably 290-300 nm.The metal barrier layer 106 and metal-nitride barrier layer 108 may bedeposited using PVD processes such as reactive sputtering processes in anitrogen-containing atmosphere.

The contact layer 110 may be a Mo layer and deposited onto themetal-nitride barrier 108 of the barrier stack 105. Alternatively,materials such as W, Ta and Ti may also be used as contact layer. Thethickness of the contact layer may be in the range of 400-1000 nm,preferably 500-900 nm.

The nucleation layer 112 may be deposited on the contact layer 110 andmay include ruthenium (Ru) or a Ru alloy. The nucleation layer forms anadditional diffusion barrier on the contact layer 110 and when formed onthe contact layer or in replacement of the contact layer, the nucleationlayer 112 increases the chemical inertness and strength of the contactlayer 110, especially when wet techniques such as electrodeposition andelectroless deposition are used to form precursor stacks. The nucleationlayer 112 also provides better nucleation capability and adhesion to thematerials deposited on it. Ru layer may be deposited by techniques suchas electroless deposition, electroplating, atomic layer deposition, CVD,MOCVD, and PVD among others. The thickness of the nucleation layer 112may be in range of 1-300 nm, preferably 5-100 nm. Thin nucleation layersare preferred for cost reduction purposes. The seed layer 114 is a thincopper layer and preferably deposited on the nucleation layer 112. Whendepositing an absorber precursor layer through a wet process such aselectroplating or electroless plating, a seed layer may be used in placeof or on top of the nucleation layer. For example, if a stack includingCu, In, and Ga films is electroplated or electroless plated on the seedlayer, the seed layer acts as an adhesion/nucleation layer on which theelectroplated metal bonds well. For example, a sputtered, CVD deposited,or ALD deposited Cu film of 2-100 nm thickness acts as an efficient seedlayer upon which the precursors comprising at least one of Cu, In and Gamay be deposited with good adhesion and morphology. The seed layer alsoimproves adhesion and uniformity of Cu(In,Ga)(Se)₂ layer or Cu(In,Ga)(S,Se)₂ formed by techniques other that electrodeposition.

As shown in FIG. 1B in side view and in 1C in perspective view, once thebase layer 100 is completed, a front side 120 including an absorberlayer, a transparent layer and a conductive grid is formed on the baseto complete a solar cell 130. The absorber layer 116 including a GroupIBIIIAVIA compound, such as Cu(In,Ga)(Se)₂ or Cu(In,Ga)(S, Se)₂, formedon the surface 115 of the seed layer 114. The absorber layer ispreferably formed using a two step process including first depositing aprecursor layer having Cu, In, Ga and Se, and optionally S, on a surface115 of the seed layer 114 (or the base 100), and second reacting theprecursor layer in a reactor at a temperature range of 300-600° C. in aninert or Se gas and optionally S gas containing atmosphere. Cu, In, Gaand Se may be electroplated to form a precursor stack including one ormore films of Cu, In, Ga and Se. Optionally, a stack including Cu, Inand Ga films may be first formed by electroplating on the base and thenone or more Se films may be vapor deposited on the previously formedstack that includes Cu, In and Ga films. In the next step, thetransparent layer 118, which may include a buffer-layer/TCO (transparentconductive oxide) stack, is formed on the absorber layer 116. Anexemplary buffer material may be a (Cd, Zn)S which is generallyelectroless deposited on the absorber layer. The TCO layer is depositedon the buffer layer and an exemplary TCO material may be a ZnO layer, anindium tin oxide (ITO) layer or a stack comprising both ZnO and ITO. Theconductive grid 122, including a bulbar 124 and conductive fingers 126,is disposed on a top surface 120 of the transparent layer 118 to collectthe current generated when the light depicted by arrows ‘L’ illuminatesa top surface 119 of the transparent layer 118.

As mentioned above, during the reaction step or other deposition stepsto form above mentioned layers, unwanted diffusion of impurities intovarious parts of the solar cell may occur. Se diffusion into stainlesssteel substrate from the absorber and Fe diffusion into the absorberlayer from the stainless steel substrate are two of the harmfuldiffusion mechanisms that the present invention attempts to eliminate.During the reaction of the absorber precursor, any unwanted Se diffusiontowards the stainless steel substrate is inhibited, more preferablysubstantially inhibited such that FeSe defects are minimized, and mostpreferably eliminated such that such that FeSe defects do not exist dueto the diffusion barrier stack disposed between the absorber layer andthe substrate. Without the barrier stack, Se forms FeSe with Fe, whichcause shunting defects. Further, Fe diffusion towards the absorber layeris again inhibited, more preferably substantially inhibited such thatFeSe defects are minimized, and most preferably eliminated such thatsuch that FeSe defects do not exist due to the same diffusion barrierstack disposed between the absorber layer and the substrate.

FIG. 2 shows an alternative base layer 200 including an intermediatelayer 205 formed on a flexible substrate 202. The flexible substrate 202includes stainless steel. The intermediate layer 205 preferably includesmultiple conductive films. In one embodiment, the intermediate layer 205includes: a diffusion barrier stack 203 having a metal barrier layer 206and a first metal-nitride barrier layer 208A, a first contact layer210A, a second metal-nitride barrier 208B, a second contact layer 210B,a nucleation layer 212, and a seed layer 214. The first metal barrierlayer may be a Cr layer, a Cr alloy layer or their multiple layers. Thefirst and second nitride barrier layers may be TiN layers. The first andsecond contact layers may preferably be Mo layers. The nucleation layeris a Ru layer or a Ru alloy, and the seed layer is a Cu layer.

In one example, the metal barrier layer 206 deposited on the stainlesssteel substrate and the first metal-nitride barrier layer 208A depositedon the metal barrier layer 206. The first contact layer 210A isdeposited onto the first metal-nitride barrier 208A. The secondmetal-nitride barrier layer 208B is deposited onto the first contactlayer 210A. The second contact layer 210B is deposited onto the secondmetal nitride barrier layer 208B. The nucleation layer 212 may bedeposited on the second contact layer 210B. The seed layer 214 ispreferably deposited on the nucleation layer 112. The additionalmetal/nitride interfaces improve the barrier properties and furtherreduce diffusion across the barrier layer. The thicknesses of the secondbilayer can be in the same range as the first. Various layers depictedin the drawings are not necessarily drawn to scale.

Although aspects and advantages of the present inventions are describedherein with respect to certain preferred embodiments, modifications ofthe preferred embodiments will be apparent to those skilled in the art.

We claim:
 1. A method of manufacturing a Cu(In,Ga) Se₂ thin-film solarcell, comprising: forming an intermediate layer on a surface of aflexible conductive substrate, the flexible conductive substrateincluding iron (Fe) species, the step of forming the intermediate layercomprising: forming a conductive barrier stack on the surface of theflexible conductive substrate, the conductive barrier stack including achromium (Cr) film applied to the surface of the flexible conductivesubstrate and a titanium nitride (TiN) film deposited on the Cr film;depositing a contact layer on the conductive barrier stack; depositing anucleation layer on the contact layer; depositing an absorber precursorincluding Cu species, In species, Ga species and Se species over thenucleation layer; applying heat to the absorber precursor to formtherefrom an absorber layer including a Cu(In,Ga) Se₂ compound, whereinthe step of forming the absorber layer causes some of the Fe species inthe flexible substrate to diffuse towards the absorber layer and some ofthe Se species in the absorber layer to diffuse towards the flexibleconductive substrate, and wherein the conductive barrier stack inhibitsdiffusion of both the iron (Fe) species and the Se species across theconductive barrier stack during the step of applying heat; disposing atransparent conductive layer on the absorber layer, the transparentconductive layer including a buffer layer deposited on the absorberlayer and a transparent conductive oxide layer formed on the bufferlayer; forming a top terminal on the transparent conductive layer,thereby resulting in the Cu(In,Ga) Se₂ thin-film solar cell; and whereinafter the step of forming, the conductive barrier stack continues toinhibit diffusion of both the iron (Fe) species and the Se speciesacross the conductive barrier stack.
 2. The method of claim 1, whereinthe contact layer includes one of molybdenum (Mo), tungsten (W) andtitanium (Ti) and tantalum (Ta).
 3. The method of claim 2 furthercomprising depositing a second titanium nitride (TiN) film on thecontact layer prior to depositing a nucleation layer on the contactlayer.
 4. The method of claim 3 further comprising depositing a secondcontact layer on the second titanium nitride (TiN) film prior todepositing a nucleation layer on the contact layer.
 5. The method ofclaim 1, wherein the conductive barrier stack is formed using a physicalvapor deposition technique.
 6. The method of claim 1, wherein the stepof forming the absorber precursor comprises: electroplating a film stackincluding at least a copper (Cu) film, an indium (In) film and a gallium(Ga) film; depositing a selenium (Se) film on the film stack using oneof an electroplating process and a vapor deposition process.
 7. Themethod of claim 1, wherein the buffer layer includes CdS and thetransparent conductive oxide layer includes ZnO.
 8. The method of claim1 further comprising depositing a seed layer over the nucleation layer,wherein the seed layer is a copper (Cu) layer.
 9. The method of claim 8,wherein the seed layer has a thickness in the range of 2-100 nm.
 10. Themethod of claim 1, wherein the chromium (Cr) film has a thickness in therange of 50-100 nm.
 11. The method of claim 1, wherein the titaniumnitride (TiN) film has a thickness in the range of 200-400 nm.
 12. Themethod of claim 1, wherein the contact layer has a thickness in therange of 400-1000 nm.
 13. The method of claim 1, wherein the nucleationlayer has a thickness in the range of 1-300 nm.
 14. The method of claim1 wherein the conductive barrier stack inhibits diffusion of both theiron (Fe) species and the Se species across the conductive barrier stacksuch that FeSe defects are eliminated.